Functions of portable electronic devices, such as cellular phones and information terminals, largely depend on those of built-in devices, which include not only semiconductors and electronic circuits but also rechargeable secondary batteries. Therefore, the built-in battery is increasingly demanded to be higher in capacity, lighter and smaller simultaneously.
The batteries of lead or nickel-cadmium, which have been used for the above purposes, are difficult to be still lighter and smaller, because of their insufficient energy density. As a result, the nickel-metal hydride battery having an energy density twice as high as that of nickel-cadmium battery and the lithium ion battery having a still higher energy density than the nickel-metal hydride battery have been developed and attracting attention.
The lithium battery is a battery of non-aqueous solution or solid electrolyte having a high energy density, and has been commercialized as the back-up power source for semiconductor memories of small electrical current and also as the power source for watches and cameras. For the battery to have wider applicable areas, such as driving and power storage, it should be further developed to a super thin lithium secondary battery which is lighter, more diversified in shape and more flexible.
However, there are many problems to be solved to improve functions and safety of the secondary battery. These problems include short circuit and ignition resulting from formation of dendrite in the lithium battery. Discovery of an electrolyte having a longer charge/discharge cycle life is one of the themes to improve reliability of the battery.
The electrolytic solution of the lithium battery is required to show ion conductivity and, at the same time, low electron conductivity to prevent short circuit and accidental discharge resulting from electrons conducted between the anode and cathode. The solid electrolyte has advantages of suffering no liquid leakage, and simplified schemes for thin film making and increasing area.
The electrolytic solution for the conventional batteries may be replaced for the battery of solid polymer electrolyte. In particular, for development of super thin film battery, which is referred to as paper battery, and electrochemical devices having a large area, such as electrochromic devices, it is necessary to develop solid polymer electrolytes for easiness of assembling and stability over extended periods.
In particular, the lithium secondary battery of solid polymer electrolyte has been attracting much attention, because of its various advantages, such as no formation of dendrite which may cause damages resulting from short circuit and ignition, no leakage of liquid unlike the case of a solution type secondary battery, and particularly ability of being made into a thin film and large area.
Some of the conventional solid polymer type solid electrolytes use a lithium salt such as LiClO.sub.4 dissolved and dispersed in a polymer, such as polyether including polyethylene oxide and polypropylene oxide, polyester, polyimide and polyether derivatives. Such an electrolyte, however, needs a sufficiently higher temperature above room temperature to exhibit its ionic conductivity of 10.sup.-5 to 10.sup.-3 S/cm.
Therefore, solidified liquid electrolytes for polymer batteries have been attracting attention, in particular those of gelled polymers with matrices impregnated with a solution similar to that for the conventional solution type lithium battery with respect to salt and solvent that dissolves it. These electrolytes include cross-linked polyalkylene oxide as disclosed by U.S. Pat. No. 4,303,748 and gelled polyacrylate as disclosed by U.S. Pat. No. 4,830,939. More recently, the technique has been developed to produce the electrolyte of polymer gel which is impregnated with a polycarbonate solution with a lithium salt dissolved in a copolymer of polyvinylidene fluoride and hexafluoropropylene, as disclosed by U.S. Pat. No. 5,296,318. These electrolytes, however, have still problems of solvent maintainability, because the electrolytic solution may ooze out at high temperature as a result of gel shrinkage.
For the conductor, a thin-film conductor (porous conducting film) having a high conductivity in spite of its high porosity can be effectively used as the electrode or as a material for the electrode of the device which involves a solid polymer or liquid electrolyte. Its high porosity provides it with a large contact interface between the electrode and electrolyte, making it suitable for primary and secondary lithium batteries of high functions.
Japanese Patent Laid-Open No.3-87096 discloses a porous conductive film and production thereof, where an electrolytic solution is immobilized by capillary condensation force on the porous thin film produced from a plasticizer solution of polyethylene mixed with Ketjen black (trade name of Akzo Chemie), which is molded into sheet, drawn and treated to remove the plasticizer. However, the problems associated with maintainability of electrolytic solution are not completely solved by this technique. More recently, the new technique has been developed to use polymer gel for the anode and cathode of the battery, where the polymer gel is impregnated with a polycarbonate solution with LiMn.sub.2 O.sub.4 and carbon black or petroleum coke and carbon black dissolved in a copolymer of polyvinylidene fluoride and hexafluoropropylene, as disclosed by U.S. Pat. No. 5,296,318. These electrolytes, however, have still problems of solvent maintainability, because the electrolytic solution may ooze out at high temperature as a result of gel shrinkage. Therefore, thin-film conductors which can be easily produced to have a large area and exhibit stable maintainability of the electrolytic solution over a wide temperature range are increasingly demanded.
The thin film has another advantage of reduced effective resistance. Japanese Patent Laid-Open No. 1-158051 discloses a technique to immobilize a liquid ion conductor using capillary condensation in the fine pores of 0.1 .mu.m or less in size in the thin, porous film of solid polymer having a thickness of 50 .mu.m or less. However, this technique by itself cannot drastically solve the problems associated with operational temperature.
It is an object of the present invention to solve the above problems, and to provide a thin film of non-protonic electrolyte which is easily produced into thin film and to have a large area, holds the solvent for the non-protonic electrolytic solution over a wide temperature range, works stably over extended periods and has improved mechanical strength; an electrolyte-immobilized liquid-film conductor; and a polymeric battery in which at least one of the thin film of non-protonic electrolyte and electrolyte-immobilized liquid-film conductor is used.